Our microbiome, the collection of microorganisms on and in our bodies, is important to our health. These microorganisms also cause healthcare associated infections. Decolonization is a rapidly growing strategy to prevent healthcare-associated infections fueled by new healthcare policy initiatives such as public reporting of healthcare associated infections and mandated surveillance cultures. Decolonization involves the application of targeted or non-targeted antimicrobials to the skin or mucosal surfaces. Given the role of our microbiome as a barrier to infection, decolonization regimens could have unintended negative consequences. Our preliminary microbial community profiling data suggests that Gram-negative bacilli (GNB) are part of the microbiome of the anterior nares, particularly in older adults. A recent meta-analysis reported that decolonization with intranasal mupirocin, a Gram-positive antimicrobial agent, increases the risk of infections due to organisms other than S. aureus including GNB. Because Gram-negative bacilli are increasingly multi-drug resistant and few novel antimicrobials are under development to treat them, interventions that shift infections from Gram-positive to Gram-negative pathogens potentially have serious negative consequences for patients. We propose to study the microbiome of the anterior nares, posterior pharynx and two skin sites in two populations with and without on-going exposure to the healthcare environment: (a) community dwelling older adults and (b) nursing home dwelling older adults. We will compare the microbial communities at baseline in both populations and then before-after a decolonization regimen within each individual. Our overall hypothesis is that the microbial composition of these sites and the response to decolonization is influenced by the healthcare environment and that decolonization leads to re-colonization with an increasing proportion of Gram- negative bacilli. This proposal links a clinically important problem, the prevention of healthcare- associated infections, with a novel methodology, microbial community profiling using high-throughout pyrosequencing of the 16S rRNA gene to answer a high impact question. Our short term goal is to determine if these increasingly used decolonization regimens, intended to control methicillin-resistant S. aureus (MRSA), result in a secondary negative effect of promoting colonization with pathogenic Gram-negative bacilli. The long term goal of this research is to use the information gained to develop new ways to manipulate the human microbiome during healthcare to reduce the risk of healthcare associated infections.